[FIX] add scrollbar to filters menu if needed (web client)

the filters menu can quickly grow if the user adds many conditions.
Once it grows out of the screen, the lower buttons can not be reached
anymore.  This patch adds a vertical scrollbar to fix this issue.  It
has to use some javascript to update its max-height property.
[ADD] website_forum_seo: add new module for improve forum que/ans description